Method for producing flocculant for water treatment

ABSTRACT

A flocculant for water treatment containing a highly polymerized silicic acid solution having a high concentration of SiO 2  and an extremely low concentration of alkali metal substance, and a method for producing such flocculant. The silicic acid solution is prepared by treating an aqueous solution of an alkali metal silicate such as water glass to remove alkali metal and allowing the solution to liquefy after once gelling. The flocculant may further contain a water soluble metal salt such as ferric chloride or ferric nitrate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a flocculant for water treatment to removeimpurities from water and, particularly, to a flocculant containing apolymerized silicic acid solution. The invention relates also to amethod for producing the flocculant.

2. Prior Art

It is widely known to use activated silica as a component of aflocculant for water treatment to purify water by removing impuritiestherefrom. Such activated or polymerized silica, however, has been usedas a mere additive to a main flocculant consisting of a metal salt suchas aluminum chloride or aluminum sulphate.

Recently, it has been proposed to use a solution of highly polymerizedsilicic acid as a flocculant for water treatment. For instance, U.S.Pat. No. 4,923,629 discloses a flocculant which comprises a solutioncontaining a highly polymerized silicic acid and a small amount of ametal salt.

One problem of a flocculant comprising a solution of highly polymerizedsilicic acid is how to increase the concentration of effectivecomponents (silicic acid and metal substance) in the solution. Namely,the higher the concentration of effective components in the flocculantsolution, the greater the capability of the flocculant for coagulatingimpurities in water. Moreover, the cost of transporting the flocculantsolution can be reduced, because the solution can be stored andtransported in a more condensed form. From a practical standpoint, it isdesirable for the concentration of SiO₂ in the flocculant solution to benot less than about 8% (by weight) and the concentration of alleffective components (SiO₂ +MeO_(x)) to be not less than 10% (byweight).

It is also well known, however, that the higher the concentration ofSiO₂ in a solution of silicic acid, the easier it is for the solution togel. A gelled solution naturally cannot be used as a flocculant.Therefore, it has not been practiced to use a silicic acid solution inwhich the concentration of SiO₂ exceeds about 8%. This is acontradictory problem in using a flocculant comprising a silicic acidsolution.

SUMMARY OF THE INVENTION

An object of the invention is to provide a flocculant for watertreatment which comprises a solution of highly polymerized silicic acidhaving a high concentration of SiO₂ and other effective components andwhich has a high capability of coagulating impurities in water and canbe safely transported and stored for a long time without gelling. It isalso an object to provide a method for producing such a flocculant.

According to the invention, a solution of highly polymerized silicicacid which has been subjected to a treatment to remove alkali metals andin which the concentration of alkali metal has been reduced to anextremely low degree, is used. Activated silica or highly polymerizedsilicic acid is usually produced from a silicate compound of an alkalimetal, such as sodium silicate or potassium silicate, by reacting itwith a compound such as sulfuric acid or carbon dioxide. Throughneutralization with the sulfuric acid or the carbon dioxide, a watersoluble salt such as Na₂ SO₄ or NaHCO₃ is produced and the alkalinity ofthe solution due to alkali metal substances in the raw material isreduced. Notwithstanding the reduction of alkalinity, however, theconcentration of alkali metal in the solution is not reduced, since analkali metal salt is still present in the solution in a dissociatedform.

The inventors have found that a solution of highly polymerized silicicacid, which is subjected to a treatment to remove alkali metals and inwhich the concentration of alkali metal substance is greatly reduced,does not gel for a surprisingly long time even if the concentration ofSiO₂ or other effective substances in the solution is high. Namely, asolution of highly polymerized silicic acid in which the concentrationof alkali metal has been extremely lowered can be used as a flocculantwhich has both high coagulating capability and high stability. Forinstance, as seen from the embodiments described hereinafter, a solutionof highly polymerized silicic acid of which the Na concentration hasbeen reduced to about 0.1% does not gel even after being stored forabout eighteen months with an SiO₂ concentration of about 9% and stillhas a very high coagulating capability. It has been found that, for thepurposes of the invention, the concentration of alkali metal in thesolution should not be more than about 0.3%. The concentration of SiO₂in the silicic acid solution of the invention can be increased togreater than about 15% without losing its high stability by using acondensing apparatus such as a rotary evaporator.

As described in U.S. Pat. No. 4,923,629, it is desirable that thesolution of highly polymerized silicic acid have a limiting viscosity ofnot less than about twice that of silicic acid monomer (that is, about0.2(100ml/g)) in order to have a high coagulating capability as a mainflocculant. Further, as with the flocculant described in the abovepatent, the silicic acid solution of the invention is usually used as aflocculant together with a small amount of a metal salt which is capableof forming a hydroxide in water such as aluminum sulphide or ferricchloride. For the purpose of further increasing the stability, a metalsalt which is capable of generating ferric ion in the solution, such asferric sulphide, ferric chloride or ferric nitrate, is preferred. Theuse of ferric nitrate is found to be especially desirable.

A preferred method for producing the flocculant of the invention is asfollows:

A raw material of an alkali metal silicate such as sodium silicate orpotassium silicate is subjected to a treatment to remove alkali metal.At this stage, it is not essential to adjust the concentration of SiO₂in the starting material to a high concentration such as 8%, although itis preferred.

Ordinarily, the treatment to remove alkali metal is carried out eitherby adding an ion exchange resin to the solution and removing ittherefrom by filtration after stirring or by passing the solution ofalkali metal silicate through a bed of ion exchange resin. By thistreatment, a large portion of alkali metal such as sodium or potassiumin the solution is removed, and a solution of silicic acid in which theconcentration of alkali metal is extremely low can be obtained. It ispreferred to adjust the concentration of alkali metal in the solution tonot more than 0.3%.

The solution of low alkali metal concentration thus obtained is oncebrought to a gel condition and, thereafter, brought again to a solutioncondition by allowing it to stand. This is an important step forobtaining the flocculant of the invention because it has been found thatonly a silicic acid solution of extremely low alkali metal concentrationcan be made into an effective flocculant through this liqueficationstep. A silicic acid solution having a higher concentration of alkalimetal can no longer be made into an effective flocculant after it gels.

In order to make the time of the step for gelation and liquefication ofthe silicic acid solution shorter, it is desirable to adjust the pH ofthe solution to a range close to neutrality, for instance, a pH of about6-10. Therefore, when the solution of low alkali metal concentrationobtained by the treatment for removing alkali metal has a low pH, it ispreferred to add a small amount of an alkaline agent such as NaOH to thesolution for the purpose of pH adjustment. The addition of a smallamount of alkaline agent such as NaOH for the purpose of pH adjustmentdoes not substantially affect the concentration of alkali metal.

Thus, the gel of silicic acid solution of low alkali metal concentrationis again converted to a liquid. For faster liquefication of the gel, itis preferred that the gel be allowed to stand for about 24 hours at atemperature of about 60° C. The viscosity of the solution obtained byliquefication is preferably adjusted so that it has a limiting viscosityof not less than 0.2(100ml/g). Further, if the concentration of SiO₂ inthe solution is still low, the solution is concentrated by using acondensing apparatus such as a rotary evaporator so that the SiO₂concentration becomes not less than 8%.

By the above process, there can be obtained a solution of silicic acidwhich has a high SiO₂ concentration, a high limiting viscosity and anextremely low concentration of alkali metal. Usually, a small amount ofmetal salt capable of forming a hydroxide in water such as ferricchloride, ferric nitrate or aluminum sulphide is further added to thissolution and the solution is used as a flocculant for water treatment.

The foregoing and other objects, features and advantages of the presentinvention will be understood more clearly and fully from the followingdetailed description of preferred embodiments.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS EXAMPLE 1

100g of an aqueous solution of "Class 3 water glass", of which the SiO₂concentration was adjusted to 8.6% by weight, was mixed with 100g of an"H type" cation exchange resin and stirred for 5 minutes. After stirringand removing the cation exchange resin by filtration, 60g of a silicicacid solution having an Na concentration of 0.02% by weight wasobtained.

The pH of the solution was adjusted to 8.8 by adding 2ml of IN-NaOHthereto and stirring. The viscosity of the solution rapidly increasedand the solution set to a gel at about 30 seconds after adding the NaOH.

The gel was again converted to a liquid by allowing it to stand for 24hours in a vessel the inside temperature of which was maintained at 60°C. By the liquefication, a solution of highly polymerized silicic acidhaving a 7.9% SiO₂ concentration and 0.10% Na concentration wasobtained. The limiting viscosity and mean molecular weight of thesolution were determined by using a Ubbelohde viscosity meter and amolecular-weight fractionation method with ultrafilters, respectively.The limiting viscosity was 0.29(100ml/g) and the mean molecular weightwas about 280,000(g/mol).

EXAMPLE 2

100g of an aqueous solution of "Class 3 water glass", of which the SiO₂concentration was adjusted to 9.0%, was mixed with 200g of an "H Type"cation exchange resin and stirred for 5 minutes. After stirring andremoving the cation exchange resin by filtration, 90g of a silicic acidsolution having a 9.0% SiO₂ concentration and 0.01% Na concentration byweight was obtained. The pH of the solution was adjusted to 8.8 byadding 3.0 ml of IN-NaOH thereto and stirring. The solution set to a gelat 30 sec. after adding the NaOH. The gel was liquefied by allowing itto stand for 24 hr. in a vessel the inside temperature of which wasmaintained at 60° C.

The solution obtained by the above liquefication of gel was furtherstored in a vessel the inside temperature of which was kept at 20° C.for 18 months. It was determined by the same methods as in Example 1that the SiO₂ concentration, Na concentration, limiting viscosity andmean molecular weight of the solution stored for 18 months wererespectively as follows:

    ______________________________________                                        SiO.sub.2          8.7%                                                       Na concentration   0.084%                                                     Limiting viscosity 0.27(100 ml/g)                                             Mean molecular weight                                                                            about 270,000(g/mol)                                       ______________________________________                                    

EXAMPLE 3

2 kg of an aqueous solution of "Class 3 water glass", of which the SiO₂concentration was adjusted to 8.6%, was mixed with 2 kg of an "H type"cation exchange resin and stirred for 5 minutes. After stirring andremoving the cation exchange resin by filtration, 1.8 kg of a silicicacid solution having a 0.02% Na concentration and 8.0% SiO₂concentration was obtained. 60 ml of IN-NaOH was added to the solutionfor the purpose of pH adjustment. The solution set to a gel at 30 sec.after adding NaOH. The gel was liquefied by allowing it to stand for 24hours in a vessel the inside temperature of which was maintained at 60°C. The silicic acid solution thus obtained by the liquefication wasfurther concentrated by using a rotary evaporator to increase theconcentration 2.5 times. (The rotary evaporator was operated at 75° C.,40rpm and 20-150mmHg.)

The SiO₂ concentration, Na concentration, limiting viscosity and meanmolecular weight of the silicic acid solution after being concentratedwere determined to be as follows:

    ______________________________________                                        SiO.sub.2 concentration                                                                          18.8%                                                      Na concentration   0.26%                                                      Limiting viscosity 0.28(100 ml/g)                                             Mean molecular weight                                                                            about 280,000(g/mol)                                       ______________________________________                                    

EXAMPLE 4

1 kg of an aqueous solution of "Class 3 water glass", of which the SiO₂concentration was adjusted to 4.3%, was passed through a bed having aheight of 32cm of an "H type" cation exchange resin which was disposedin a filter cylinder having an inside diameter of 5 cm. with a passingrate of 2 m/hr. 900 g of filtrate having an SiO₂ concentration of 3% andan Na concentration of 0.004% was obtained. To the filtrate was added 8ml of IN-NaOH with stirring by using a magnetic stirrer so that the pHwas adjusted to 8.8%. The solution was allowed to stand at a temperatureof 20° C. The viscosity of the solution gradually increased, and thesolution set to a gel at 13 hours after the start of standing. The gelwas liquefied by further allowing it to stand for 24 hours in a vesselthe inside temperature of which was maintained at 60° C. The liquefiedsolution was concentrated by using a rotary evaporator to increase theconcentration two times. (The rotary evaporator was operated at 75° C.,40 rpm and 20-150 mmHg.)

The SiO₂ concentration, Na concentration, limiting viscosity and meanmolecular weight of the concentrated solution were determined to be asfollows:

    ______________________________________                                        SiO.sub.2 concentration                                                                          8.6%                                                       Na concentration   0.048%                                                     Limiting viscosity 0.23(100 ml/g)                                             Mean molecular weight                                                                            about 260,000(g/mol)                                       ______________________________________                                    

EXAMPLE 5

500g of an aqueous solution of potassium silicate, of which the SiO₂concentration was adjusted to 8.6%, was mixed with 500 g of "H type"cation exchange resin and stirred for 5 minutes by using a magneticstirrer. Then, after removing the cation exchange resin by filtration,14 ml of 1N-NaOH was added to the solution. The viscosity of solutionrapidly increased and the solution gelled at sec. after the addition ofNaOH. The gel was liquefied by allowing it to stand for 24 hours in avessel the inside temperature of which was maintained at 60° C.

The SiO₂ concentration, potassium concentration, limiting viscosity andmean molecular weight of the liquefied solution were determined to be asfollows:

    ______________________________________                                        SiO.sub.2 concentration                                                                          8.4%                                                       K concentration    0.076%                                                     Limiting viscosity 0.29(100 ml/g)                                             Mean molecular weight                                                                            about 280,000(g/mol)                                       ______________________________________                                    

EXAMPLE 6

By combining the polymerized silicic acid obtained in Example 3 withthree different metal salts (ferric chloride, ferric nitrate and ferricsulphide), twelve samples of flocculants having a total concentration ofeffective components (SiO₂ +Fe₂ O₃) of 10.0%, 12.5%, 15.0% or 17.5% wereprepared as follows: The solution of polymerized silicic acid preparedin Example 3 was divided into twelve samples (Samples 1 through 12). Toeach sample distilled water and metal salt were added. That is, ferricchloride was added to four samples of a first group (Samples 1 to 4ferric nitrate was added to four samples of a second group (Samples 5 to8) and ferric sulphide was added to four samples of a third group(Samples 9 to 12), so that the mol ratio of Si:Fe in each sample was3:1.

EXAMPLE 7

The solution of polymerized silicic acid obtained in Example was dividedinto three samples (Samples 13 to 15). To each sample was addeddistilled water and a different metal salt, that is, Sample 13 (ferricchloride), Sample 14 (ferric nitrate) and Sample 15 (ferric sulphide),so that the mol ratio of Si:Fe and the total concentration of effectivecomponents (SiO₂ +Fe₂ O₃) in each sample was 3:1 and 10%, respectively.

EXAMPLE 8 (FOR COMPARISON)

Three samples for comparison were prepared by using a highly polymerizedsilicic acid solution which was not subjected to a treatment to removealkali metal and has a high total concentration of effective components,as follows:

640 g of an aqueous solution of "Class 3 water glass", of which the SiO₂concentration was adjusted to 11.0%, was mixed with 160 ml of 5.4N-HClwith stirring. 800 g of a silicic acid solution having a pH of 2.0 andSiO₂ concentration of 8.8% was obtained. The solution was polymerized byslowly stirring for 30 minutes in a vessel the temperature of which wasmaintained at 60° C. A solution of polymerized silicic acid having alimiting viscosity of 0.30(100 ml/g) and a mean molecular weight ofabout 350,000(g/mol) was obtained.

The silicic acid solution thus obtained, of which the Na concentrationwas not reduced, was divided into three samples (Samples 16 to 18). Toeach sample was added a different metal salt, that is, Sample 16 (ferricchloride), Sample 17 (ferric nitrate) and Sample 18 (ferric sulphide),so that the mol ratio of Si:Fe in each sample was 3:1 and the totalconcentration of effective components (SiO₂ +Fe₂ O₃) was 10%.

TEST EXAMPLE 1

Eighteen samples of flocculants (Samples through 18) prepared in Example6, Example 7 and Example 8 were subjected to a test for theirdetermining gel times by keeping them in two vessels the temperatures ofwhich were maintained constant at 60° C. and 20° C., respectively.

The result of the test is mentioned in Table 1.

                                      TABLE 1                                     __________________________________________________________________________    Sample                                                                            Added  Mol. Ratio                                                                          Con. (%)   Gel Time                                                                           (hr.)                                        No. Salt   (Si:Fe)                                                                             (SiO.sub.2 + Fe.sub.2 O.sub.3)                                                        pH (60° C.)                                                                    (20° C.)                              __________________________________________________________________________     1  FeCl.sub.3                                                                           3:1   10.0    1.5                                                                              110  8800                                          2    "    "     12.5    1.3                                                                              95   6000                                          3    "    "     15.0    1.3                                                                              50   2650                                          4    "    "     17.5    1.2                                                                              30   1100                                          5  Fe(NO.sub.3).sub.3                                                                   "     10.0    1.1                                                                              360  >12000                                        6    "    "     12.5    0.9                                                                              190  8800                                          7    "    "     15.0    0.8                                                                              90   3850                                          8    "    "     17.5    0.6                                                                              48   2400                                          9  Fe(SO.sub.4).sub.3                                                                   "     10.0    1.3                                                                              35   1100                                         10    "    "     12.5    1.1                                                                              20    120                                         11    "    "     15.0    1.0                                                                              10    50                                          12    "    "     17.5    0.9                                                                               8    25                                          13  FeCl.sub.3                                                                           "     10.0    1.4                                                                              95   >4300                                        14  Fe(No.sub.3).sub.3                                                                   "     "       1.1                                                                              240  >4300                                        15  Fe(SO.sub.4).sub.3                                                                   "     "       1.3                                                                              35   1100                                         16  FeCl.sub.3                                                                           "     10.0    1.5                                                                               2    30                                          17  Fe(NO.sub.3).sub.3                                                                   "     "       1.1                                                                               4    50                                          18  Fe(SO.sub.4).sub.3                                                                   "     "       1.3                                                                                0.5                                                                                6                                          __________________________________________________________________________

As can be seen from Table 1, the flocculants of the invention containingthe silicic acid solution which was subjected to the treatment forremoving alkali metral (Sampels 1 to 15) showed excellent stabilitywhere the total concentration of effective components was 10% or more,while the flocculants containing the silicic acid solution which was notsubjected to such an alkali metal removing treatment (Samples 16 to 17)had gel times of not more than several tens of hours at 20° C. when theconcentration of effective components was 10%. Especially, theflocculants of the invention which contain FeCl₃ or Fe(NO₃)₃ as a metalsalt remained stable without gelling for about 1000 to 4000 hours at 20°C. even when the total concentration of effective components was in avery high range such as 15.0 to 17.5%.

Then, the flocculants of Samples 1 to 15 were subjected to jar test, forthe purpose of confirming the coagulating capability, as follows:

Each sample of flocculant was poured with stirring into test waterhaving a turbidity of 100 degrees, alkalinity of 60 mg/1 (as CaCO₃) anda pH of 7.5 which was prepared by adding kaolin and NaHCO₃ to distilledwater. The flocculant was used at a rate of 2.0 mg/1 as Fe and stirredfor 7 minutes at a speed of 120 rpm (at starting) and 30 rpm (at 3minutes after starting). The time when flocus appeared after the startof stirring, the average size of flocs and the turbidity of the treatedwater are shown in Table 2.

                  TABLE 2                                                         ______________________________________                                        Sample                                                                              Floc Appearance                                                                            Average      Water Turbidity                               No.   Time (sec.)  Floc Size (mm)                                                                             (degree)                                      ______________________________________                                         1    30           2.0-5.0      0.6                                            2    30           2.0-5.0      0.8                                            3    30           2.0-5.0      0.6                                            4    30           2.0-5.0      0.8                                            5    30           2.0-5.0      0.6                                            6    30           2.0-5.0      0.8                                            7    30           2.0-5.0      0.8                                            8    30           2.0-5.0      0.7                                            9    30           2.0-5.0      0.7                                           10    30           2.0-5.0      0.8                                           11    30           2.0-5.0      0.6                                           12    30           2.0-5.0      0.8                                           13    30           2.0-5.0      0.7                                           14    30           2.0-5.0      0.9                                           15    30           2.0-5.0      0.6                                           ______________________________________                                    

As can be seen from Table 2, it was confirmed that the flocculants ofthe invention showed excellent results in each of floc appearance time,floc size and water turbidity after treatment.

TEST EXAMPLE 2

The flocculant obtained in Example 2 which was stored for 18 monthsafter preparation was subjected to a jar test for confirming itscoagulating capability after long term storage, as follows:

100 g of a flocculant of which the mol ratio of Si:Fe was 3:1 and thetotal concentration of effective components (SiO₂ +Fe₂ O₃) was 10%, wasprepared by adding 10.4 g of ferric chloride (feCl₃.6H₂ O) and water tothe solution of polymerized silicic acid obtained in Example 2.

The flocculant was poured into a test water having a turbidity of 100degrees, an alkalinity of 60 mg/1 (as CaCO₃) and a pH of 7.5, which wasprepared by adding kaolin and NaHCO₃ to distilled water.

The pouring rate of flocculant was 20 mg/1 as Fe and stirring wascarried out for 7 minutes at a rate of 120 rpm (at starting) and 30 rpm(3 minutes after starting). The floc appearance time, the average flocsize and the turbidity of treated water at 5 minutes after the stop ofstirring were as follows:

    ______________________________________                                        Floc Appearance time  30(sec)                                                 Average floc size     2.0-5.0(mm)                                             Turbidity of treated water                                                                          0.6 (degree)                                            ______________________________________                                    

Thus, it was confirmed that the flocculant prepared in Example 2 andstored for 18 months also has excellent coagulating , capability.

TEST EXAMPLE 3

A test was carried out for the purpose of confirming the

ing capability of a flocculant which comprises a polymerized silicicacid solution prepared by a process in which the addition of an alkalineagent for the purpose of adjusting pH was not made, as follows:

100 g of an aqueous solution of "Class 3 water glass" of which theconcentration of SiO₂ was 8.6%, was mixed with 50 g of an "H type"cation exchange resin and stirred for 5 minutes with a magnetic stirrer.90 g of silicic acid solution having an 8.6% SiO₂ concentration, 0.14%Na concentration and pH of 9.3, was obtained by removing the cationexchange resin by filtration after the finish of stirring. The obtainedsolution gelled after about 10 minutes, at a room temperature of 20° C.The gel was liquefied by allowing it to stand for 24 hours in a vesselthe inside temperature of which was constantly maintained at 60° C. Thesolution of polymerized silicic acid thus obtained had a limitingviscosity of 0.23(100 ml/g) and a mean molecular weight of about240,000.

100g of a flocculant of which the mol ratio of Si:Fe was 3:1 and thetotal concentration of effective components (SiO₂ +Fe₂ O₃) was 10%, wasprepared by adding 10.4g of ferric chloride (FeCl₃.6H₂ O) and water to80.6g of the above solution of polymerized silicic acid.

The coagulating capability of the flocculant was determined by a jartest. Namely, the flocculant was poured into a test water having 100degrees of turbidity, 60 mg/1(as CaCO₃) of alkalinity and a pH of 7.5prepared by adding kaolin and NaHCO₃ to distilled water, and stirred.The rate of pouring of the flocculant was 2.0mg/1 and the stirring wascarried out for 7 minutes at a rate of 120 rpm (at the start) and 30rpm(at 3 minutes after starting). The floc appearance time, the averagefloc size and the turbidity of the treated water were as follows:

    ______________________________________                                        Floc appearance time  30 (sec)                                                Average floc size     2.5-5.0(mm)                                             Water turbidity       0.8 (degree)                                            ______________________________________                                    

Thus, it was confirmed that the flocculant prepared by the above processhas excellent coagulating capability.

TEST EXAMPLE 4

The following test was carried out for the purpose of clarifying thevariation of characteristics of the polymerized silicic acid solutionwhen the alkali metal concentration in the solution is varied.

2kg of an aqueous solution of "Class 3 water glass", of which the SiO₂concentration was adjusted to 9.0%, was subjected to treatment to removealkali metal by mixing it with 3 kg of an "H type" cation exchange resinand stirring for 5 minutes.

After removing the cation exchange resin by filtration, 1.8 kg of asilicic acid solution having a 9.0% SiO₂ concentration and 0.01% Naconcentration was obtained. The solution was divided into 11 samples.After adding different amounts of sodium hydrate to each sample, theirpH, SiO₂ concentration and Na concentration were determined. The sampleswere allowed to stand at room temperature of 20° C. to determine theirgel time. Eight samples gelled within 30 minutes and three samples didnot gel even after a lapse of 24 hours. Both the gels and the solutionswhich did not gel were further allowed to stand for 24 hours in a vesselthe temperature of which was maintained constant at 60° C. After thelapse of 24 hours from the start of standing at 60° C., it was observedthat the eight gel samples had converted to a liquid and the other threesolution samples set to gel. The liquefied samples were analyzed todetermine their limiting viscosity and mean molecular weight. Theresults are shown in Table 3.

                  TABLE 3                                                         ______________________________________                                                                                   Mean                               Sam- SiO.sub.2                                                                            Na     GEL time                                                                              Condition                                                                             Limiting                                                                              mol.                               ple  con.   con.   at 20° C.                                                                      after 24 hr                                                                           Vis.    Weight                             No.  (%)    (%)    (min.)  at 60° C.                                                                      (100 ml/g)                                                                            (g/mol)                            ______________________________________                                        1    8.6    0.041   1      solution                                                                              0.48    640000                             2    8.6    0.051   1      solution                                                                              0.42    580000                             3    8.6    0.062   1      solution                                                                              0.37    500000                             4    8.6    0.108   1      solution                                                                              0.23    260000                             5    8.6    0.154  10      solution                                                                              0.21    220000                             6    8.6    0.l85  10      solution                                                                              0.20    200000                             7    8.6    0.200  30      solution                                                                              0.18    170000                             8    8.6    0.291  30      solution                                                                              0.14     90000                             9    8.6    0.449  not gelled                                                                            gel     --      --                                                    in 24 hr                                                   10   8.6    0.514  not gelled                                                                            gel     --      --                                                    in 24 hr                                                   11   8.6    0.980  not gelled                                                                            gel     --      --                                                    in              24 hr                                      ______________________________________                                    

As seen from Table 3, it was observed that the solutions having an Naconcentration exceeding about 0.3% did not show the characteristic ofconverting from a gel to a liquid necessary for the present invention.

It should be understood that, although the preferred embodiments of thepresent invention have been described herein in considerable detail,certain modifications, changes, and adaptations can be made by thoseskilled in the art and that it is hereby intended to cover allmodifications, changes and adaptations thereof falling within the scopeof the appended claims.

What is claimed is:
 1. A method for producing a flocculant for watertreatment consisting essentially of:subjecting an aqueous solution ofwater glass to a cation treatment to remove alkali metal, polymerizingsaid treated solution to obtain a gel-like silicic acid solution,allowing said gel-like silicic acid solution to stand for a periodsufficient to change it again to a liquid solution having a limitingviscosity of not less than about 0.2 (100 ml/g), and adjusting theconcentration of SiO₂ of said liquid solution to a value of not lessthan about 8%.
 2. The method for producing a flocculant for watertreatment according to claim 1, further comprising mixing the liquidsolution having a limiting viscosity of not less than about 0.2 (100ml/g) and a SiO₂ concentration of not less than about 8% with a watersoluble salt of metal capable of forming a hydroxide in water.